1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet used as a soft magnetic material for an iron or magnet core of electrical equipments.
2. Description of the Related Art
A grain-oriented electrical steel sheet has a crystal grain orientation referred to as "Goss-orientation", in which grains are {110}&lt;001&gt;-oriented in terms of the Miller index, and usually has a Si content of 4.5% or less and a sheet thickness of from 0.10 to 0.35 mm. The steel sheet should have an excellent magnetic characteristic, particularly the magnetic flux density and the watt-loss characteristics and, to meet that requirement, it is important that the crystal grains are highly uniformly aligned in the Goss-orientation. This extremely high accumulation to the Goss-orientation is achieved by utilizing a catastrophic grain growth referred to as "secondary recrystallization". To control the secondary recrystallization, it is indispensable to adjust the primary-recrystallized structure prior to the secondary recrystallization and also to adjust the fine precipitates referred to as inhibitors or the elements segregating on the grain boundaries. The inhibitor suppresses the growth of the primary-recrystallized grains which are out of the Goss-orientation, and thereby, promotes the preferential growth of grains which are in the Goss-orientation.
Typical precipitates are MnS as proposed by M. F. Littman in Japanese Examined Patent Publication (Kokoku) No. 30-3651 or by J. E. May and D. Turnbull in Trans. Met. Soc. A.I.M.E. 212, 1958, p769-781, AlN as proposed by Taguchi and Sakakura in Japanese Examined Patent Publication (Kokoku) No. 40-15644, MnSe as proposed by Imanaka et al. in Japanese Examined Patent Publication (Kokoku) No. 51-13469, and (Al, Si)N as proposed by Komatsu et al in Japanese Examined Patent Publication (Kokoku) No. 62-45285.
The elements segregating on the grain boundaries are Pb, Sb, Nb, Ag, Te, Se, S, etc., as reported by Saito et al. in Journal of the Japan Institute of Metals, 27, 1963, p186-195 but these elements are used as merely an assistive agent for the precipitate inhibitors in the industries.
Although the essential conditions under which such precipitates can function as an inhibitor have not yet been fully clarified, an explanation was proposed by Matsuoka in Tetsu-to-Hagane (Iron and Steel) 53, 1967, p1007-1023 or by Kuroki et al. in Journal of the Japan Institute of Metals, 43, 1979, p-175-181 and ibid, 44, 1980, p-419-424, as summarized below.
(i) Fine precipitates should be present in an amount sufficient to suppress the growth of the primary-recrystallized grains prior to the secondary recrystallization.
(ii) Precipitates should have a certain size and should not abruptly vary by heat during annealing for effecting the secondary recrystallization.
The processes currently used in the manufacture of a grain-oriented electrical steel sheet are generally classified in the following three types.
The first type utilizes a two-step cold rolling using MnS disclosed by M. F. Littman in Japanese Examined Patent Publication (Kokoku) No. 30-3651, the second type utilizes a large reduction of 80% or more in the final cold rolling step using AlN and MnS disclosed by Taguchi and Sakakura in Japanese Examined Patent Publication (Kokoku) No. 40-15644, and the third type utilizes a two-step cold rolling using MnS (or MnSe) and Sb disclosed by Imanaka et al. in Japanese Examined Patent Publication (Kokoku) No. 51-13469.
These processes commonly use a basic technology in which a steel slab is heated at a high temperature in the hot rolling step so that an in-situ formation of inhibitors is effected to ensure the necessary precipitate amount and also to refine the precipitates.
Namely, a steel slab is heated at a high temperature, such as 1260.degree. C. or higher in the first type process, 1350.degree. C. or higher in the second type process when the slab contains 3% Si as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 48-51852 although the temperature varies with the silicon content, or 1230.degree. C. or higher in the third type process as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 51-20716 including an example in which an extremely high temperature of 1320.degree. C. is adopted to obtain a particularly high flux density. Under such a high temperature of slab heating, coarse precipitates present in steel matrix are once dissolved in steel to form a solid solution and then a fine precipitation occurs during hot rolling and/or the subsequent heat treatment.
Control of these precipitates, however, is very difficult and various solutions to this problem have been proposed.
Japanese Examined Patent Publication (Kokoku) No. 54-14568 proposed that chromium nitride, titanium nitride, vanadium nitride or the like is added to an annealing separator to ensure the nitrogen partial pressure in the atmosphere during final annealing in which the secondary recrystallization is effected and Japanese Examined Patent Publication (Kokoku) No. 53-50008 proposed that a sulfide such as Fe.sub.2 S is added to ensure the sulfer partial pressure and suppress decomposition of the precipitates so that the secondary recrystallization is stabilized.
Nevertheless, these solutions could not enable the production of a product having an optimum magnetic characteristic.
This is essentially because it is actually impossible in the industrial practice that precipitates of a fixed size are dispersed in a fixed amount over the length and the width of a steel sheet coil by the slab heating and that this precipitation condition is kept unvaried until the secondary recrystallization begins.
The precipitation occurs under a non-equilibrium condition and is strongly affected by the prior heat and strain history. In fact, different portions of a steel slab have different heat and strain histories and a steel slab per se has a nonuniform crystal structure due to a macro-segregation of component elements over the slab thickness and to a local dispersion of the .alpha.- and the .gamma.-phases.
Therefore, the process for producing a grain-oriented electrical steel sheet based on the control of inhibitor is not essentially stable when used in industry.